1. Field of the Invention
This invention relates to microelectronic devices and the manufacturing thereof, including, but not limited to, the manufacturing of field emission, or effect, display (FED) devices. More particularly, this invention relates to the screen printing of screen printable substances onto various substrates to form, for example, electrically conductive traces, or conductor elements, on selected components of microelectronic devices such as, but not limited to, substrates incorporated within FED devices.
2. State of the Art
Screen printing is frequently used within the microelectronic industry in the manufacturing of a wide variety of microelectronic components and products. For example, various electrical circuits, or traces, can be formed on a selected planar, rigid substrate by screen printing to provide a wide selection of electrical circuitry and circuit functions. Such screen printed electrical circuits can include, for example, conductive elements and paths, resistive elements and paths, as well as various elements that have certain preselected insulative or dielectric characteristics or qualities. Thus, the term xe2x80x9cconductivexe2x80x9d as used herein broadly refers to any material capable of conducting electricity.
In the fabricating of field emission displays, or flat-panel displays, the microelectronic industry faces a constant demand by the market to make such displays thinner and lighter and generally more compact compared to the previous generation of displays. Furthermore, there is considerable market pressure for manufacturers to generally make microelectronic devices, including field emission displays, for example, more quickly and less expensively in order for companies selling products incorporating such microelectronic devices to be, and remain, competitive in the marketplace.
U.S. Pat. Nos. 5,766,053 and 5,537,738 each issued to Cathey et al., assigned to the present assignee, and which are incorporated by reference herein, disclose an exemplary internal flat-panel field emission display and exemplary methods of attaching and electrically connecting inwardly facing planar substrates having matching patterns of bond pads, respectively. In both of these patents, selected elevated bond pads located on top of an insulative spacer, or ridge, which is provided along a selected edge of the lower-most substrate, are electrically connected by wire bonds to respectively associated circuit traces which were previously disposed upon the lower-most substrate so as to terminate short of the insulative spacer and be adjacent and located below the respectively connected elevated bond pads. In both patents, the respective electrical traces and the insulative spacer, or ridge, were formed by the screen printing of conductive and dielectric screen printable materials.
Exemplary prior known screen printing processes used in the formation of microelectronic components include the printing of conductive layers upon a selected substrate by forcing a paste, or printable substance, of a preselected viscosity through a stainless steel or, more often, a monofilament polymer screen of a preselected mesh having a preselected negative pattern formed through the screen by various known methods. The screen having a preselected pattern preformed therethrough is stretched so as to be tautly secured to a support frame such that the screen and the substrate can eventually be brought into very close proximity, preferably just short of actual direct contact with each other. Upon the screen being precisely positioned above the substrate in which the screen printable substance is to be disposed, the screen printable substance is typically introduced on top of the screen and a squeegee, or rubber blade, is biased toward the substrate and is swept across the flexible screen thereby pushing the printable substance forward along the screen as well as forcing a portion of the screen printable substance downward through the negative pattern provided on the screen and onto the underlying substrate. After the printable substance has been disposed on the receiving surface of the substrate and the screen and squeegee have been lifted away therefrom, the screen printed substance, or paste, is typically dried by firing at a selected temperature and duration. Thereafter, the substrate can be readied for further screen printing. For example, a dielectric layer may subsequently be screen printed on top of an underlying, previously screen printed conductive layer, or upon the last screen printed substance being fired, and the screen printed substrate may be forwarded on for further post-screen printing processing.
With respect to the fabrication and operation of field emission displays in particular, typically, a cathode plate having a plurality of individual cathodic electrodes is positioned in a parallel, spaced apart relationship with a transparent display substrate covered by a phosphorous film acting as an anode plate. Borosilicate glass is often selected as a transparent substrate due to it having a compatible coefficient of thermal expansion and suitable structural characteristics. The two plates are spaced away from each other by at least one dielectric spacer, ridge, or rail, which borders at least a portion, if not the entire periphery, of what is to be the display area or window. Upon providing electrical potentials of appropriate polarization and magnitude to various electrodes located on the cathode plate, electrons are emitted therefrom and are drawn toward the opposing, but spaced-apart, glass substrate serving as an anode plate whereon images can be viewed through the display window. In order for the opposing cathode plate and the transparent glass substrate/anode plate to function properly, the very small space between the two plates must be uniform and the various thickness of each of the various layers of screen printed material provided on each plate must be controlled within strict dimensional tolerances. Such strict dimensional tolerances are needed, not only for keeping the final FED unit as thin as possible, but are also needed for quality control purposes of the image to be displayed. For example, various qualities of the displayed image, such as overall image uniformity, resolution, and brightness, can be directly influenced by minute, or out of specification, spacing of the two opposing plates.
U.S. Pat. No. 5,612,256 issued to Stansbury, incorporated by reference herein, is directed toward multi-layer electrical interconnection structures and fabrication methods. More particularly, the ""256 Stansbury patent discloses a flat-panel field emission display wherein a dielectric connector ridge having a generally planar top surface with generally curved side surfaces, is screen printed onto the rear surface of a faceplate of an FED device. The faceplate is also provided with a plurality of lower-level electrically conductive connectors by way of conventional screen printing that extend generally perpendicular to, and are spaced along one side but terminate short of, the dielectric connector ridge. Preferably, a plurality of discrete upper-level connectors ultimately positioned in registry with the lower-level connectors are screen printed atop the dielectric connector ridge in a subsequent screen printing process. In due course, each of the upper-level connectors, and the corresponding discrete lower-level connectors, are, respectively, electrically interconnected by a bond wire, for example, in accordance with a preferred embodiment disclosed therein.
Such a representative wire bonded connection in the context of a representative portion of an anode plate 16 of a field emission display is shown in drawing FIGS. 1A through 1C of the present drawings. More particularly in drawing FIG. 1A hereof, anode plate 16 has a transparent glass substrate 2 serving as an anode baseplate. Mounted upon substrate 2 is a first layer of a dielectric material 4. Mounted on top of dielectric layer 4 is an optional second dielectric layer 6 that is usually precision trimmed or polished to provide an upper planar surface that is of a specific height above the substrate, typically on the order of 10 mils (0.010 inches/0.254 mm) in height. Thus, dielectric layers 4 and 6 taken together, form a dielectric or insulative ridge 3, also referred to as an insulative spacer or rail. Lower level conductive element or trace 8 is located on substrate 2. Lastly, a bond wire 12 is bonded at bond points 14 to provide an electrically conductive path between lower-level conductive trace 8 and upper-level conductive trace 10.
Illustrated in drawing FIGS. 1B and 1C hereof is the screen printing process of forming conductive traces 8 and 10 on a portion of a representative substrate, which in the case of an FED serves as an anode plate 16 shown in drawing FIG. 1A. In drawing FIG. 1B, the ridge or spacer 3, comprising vertically stacked dielectric layers 4 and 6, has previously been formed onto substrate 2 by screen printing processes known within the art. A screen printing apparatus 18, including a screen support frame 20 and a flexible screen 22, is biased toward substrate 2 by a squeegee 24. The arrow depicts the direction in which squeegee 24 is moved across the top of screen 22, usually at a constant speed, thereby forcing conductive paste 26 downward through a pattern in screen 22 and onto the exposed surface of substrate 2, thus forming lower-level conductive trace 8. Illustrated in drawing FIG. 1C is the forming of upper-level conductive trace 10 by squeegee 24 biasing screen 22 downward to nearly press against the top of layer 6 while simultaneously moving forward, thereby causing conductive paste 26 to be laid down on the exposed surface of layer 6 through a preformed pattern in screen 22. Note that conductive trace 8 stops short of the proximate edges of dielectric layers 4 and 6 which form elevated ridge or rail 3 so that screen 22 does not unduly contact ridge 3 while forming lower-level conductive trace 8.
Although the ""256 Stansbury patent depicts in drawing FIG. 6 thereof, and discusses in column 5 of the specification thereof, that a continuous terminal conductor having a lower-level base portion positioned directly on the rear surface of the faceplate, and an upper-level connecting portion positioned atop the dielectric connector ridge, can be screen printed in a continuous manner onto both surfaces, the specification in column 8 states that, in practice, it is impractical to screen print such continuous terminal conductors over the abrupt elevational change presented by the connector ridge. It is also noteworthy that the connector ridge depicted in drawing FIG. 6 of the ""256 patent has a rounded or curved side profile and, clearly, does not include a substantially abrupt vertical, or substantially straight, side profile extending perpendicular to substrate 2.
Thus, there remains a need within the art for effective, practical screen printing processes and apparatus that can be used by the art to screen print screen printable substances, such as electrically conductive pastes, to form small, dimensionally close-toleranced continuous multi-level conductive traces, or conductive elements, especially suitable for use in the manufacturing of microelectronic devices, such as field emission display devices manufactured on high-speed production lines.
There further remains a need within the art for effective, practical screen printing processes and apparatus that can be used to form multi-level conductive traces, or conductive elements, suitable for use in the fabrication of microelectronic devices which require less time and fewer fabricating steps, thereby lowering the costs associated with manufacturing microelectronic devices such as field emission displays.
A still further need within the art includes the need for microelectronic devices and products which incorporate components having screen printable substances disposed thereon by screen printing processes and apparatus that offer enhanced versatility and capability compared to prior known screen printing processes and apparatus.
The present invention provides the ability to form, to close dimensional tolerances and geometries, electrically conductive traces or other structures that extend from one level to at least one other elevated level by the screen printing of screen printable substances, such as, but not limited to, conductive pastes of preselected viscosities. Preferably, the subject invention includes the screen printing of a screen printable material upon a generally planar substrate to form a conductive trace thereon. The screen printing continues in an xe2x80x9cuphillxe2x80x9d manner to extend the conductive trace upward onto at least one elevated surface located above the underlying substrate. The present invention is particularly suited for, but not limited to, the formation of multi-level conductive traces used in providing an electrically conductive path from a first level to at least one second elevated level in microelectronic devices.
The present invention is particularly useful in the fabrication of flat-panel field emission displays (FED) in which a first transparent substrate made of borosilicate glass is provided with an insulative structure or spacer, also referred to as a ridge, rail, or similar structure, made of a preselected dielectric material. The insulative spacer can extend upwards of 10 mils (0.010 inches/0.025 cm) from the underlying glass substrate. In the preferred embodiment, a continuous conductive trace having a preselected geometry, such as a generally rectangular shape, is applied to the substrate by way of a squeegee being biased against and traversing a screen having preformed patterns, or openings, therein. Preferably, the screen is very thin in cross-sectional thickness, of the magnitude of 0.2 mils (0.0002 inches/0.0005 cm) for example, and when finally positioned, is preferably positioned to have a preferred snap-off distance, of the magnitude of 0.1 to 0.125 mils (0.0001 inches/0.0003 cm to 0.000125 inches/0.00037 cm) for example, being maintained between the bottom of the screen and the top of the substrate or other surface in which the screen printable material is to be disposed upon. A very soft squeegee, that is, a wiper or blade having a comparatively low durometer value, is used in combination with the thin screen to sweep the screen printable substance of a preselected viscosity through the screen and onto the substrate and up onto the top of the spacer in preferably a continuous uninterrupted fashion to preferably form a discrete, continuous bi-level or multi-level conductive trace, or another similarly formed structure.
Preferably, the angle of the screen with respect to the top surface of the spacer is maintained at a preselected angle to optimize the disposing of the screen printable material onto the substrate and up onto the various elevated surfaces or levels that the screen printable material is to be disposed.
Furthermore, the screen used in disposing screen printable material in accordance with the present invention is preferably provided with openings, or patterns, that are geometrically configured to compensate for the xe2x80x9cuphillxe2x80x9d portion or region of the structure to be formed. For example, if a conductive trace is to have a generally constant width along its longitudinal axis, including that portion of the trace which rises from a first level to a second higher or elevated level, it may be necessary to reduce the width of the corresponding opening in the screen to compensate for distortions that may occur in the transition from one level to the next level of the conductive trace to be formed. To illustrate, it may be necessary to reduce the width of the opening in the screen corresponding to the xe2x80x9cuphillxe2x80x9d portion of the conductive trace to compensate for the screen printable material""s propensity to undesirably disperse laterally beyond the desired width that the xe2x80x9cuphillxe2x80x9d portion of the conductive trace is to have. In other words, the screen printable material or paste may flow outwardly or bulge on one or both sides of the xe2x80x9cuphillxe2x80x9d region and thereby possibly come into contact with proximately located conductive traces if the corresponding portion of the opening in the screen is not reduced in width to compensate for the tendency to bulge or spread. This unwanted lateral distortion could be particularly troublesome when using materials or pastes of high viscosity to form traces or other structures that are to be very closely positioned with respect to each other. Such a case could occur when forming thick film conductive traces that are to have a center-to-center spacing or pitch, ranging in the magnitude of a few mils to 10 mils (0.010 inches/0.254 mm) or more.
The uphill screen printing of the present invention is particularly suitable for simultaneous formation of conductive traces on several areas of a common substrate in a high-quantity, high-speed production environment in which the substrate will eventually be segmented into a multitude of individual microelectronic device sub-components. For example, a preselected number of individual areas preferably arranged in an array of a selected pattern on a substrate, such as by a preselected number of rows and columns, can have a number of screen printing operations performed thereon, including the screen printing of conductive traces or other structures, in accordance with the present invention. Upon the completion of the last operation that is to be performed on each of the individual areas or array of areas located on the common substrate, the individual areas of the array are then segmented into individual substrates which will eventually serve as an individual component in a FED device, for example.